Coatings based on a combination of epoxy resins and amine hardeners (curing agents) which react to form a crosslinked film have enjoyed widespread use for decades. Because of the combination of properties achievable they have developed strong market positions in those applications where a high degree of resistance to water, chemical reagents, or corrosive environments is required.
A good introduction to the general chemistry of epoxy resins is available in H. Lee and K. Neville, "Handbook of Epoxy Resins" (1967, McGraw-Hill Inc.). Commercially available epoxy resins useful in coatings are frequently referred to as either liquid resin or solid resin. The commercially important solid epoxy resins have an epoxy equivalent weight (EEW) greater than about 450. Although much higher EEW epoxy resins are available, the resins employed in amine cured coatings generally have an EEW less than about 1000. At higher equivalent weights the resulting crosslink density is too low to give the desired properties. Commercially important liquid epoxy resins have an EEW of less than about 250, and more frequently less than about 200. Though much slower to dry than solid epoxies, they result in films with very high crosslink densities, and find utility where very chemically resistant coatings are required. Of course, they also require less solvent for application than traditional solvent borne formulations. There is also a class of epoxy resins sometimes referred to as semi-solid resins, with EEWs intermediate between liquid and solid. It should be realized that a reference to `liquid`or `solid` resin may refer not to the actual physical state of the resin, but to the resin's EEW range, and perhaps to the properties that may be anticipated with its use. Thus, an aqueous dispersion of an epoxy resin with an EEW of 500 may be referred to as a solid resin dispersion, even though it is in a liquid form.
Concerns over environmental pollution and the health risks associated with chemical exposure have resulted in an intense effort by coatings manufacturers and raw material suppliers to develop products that have lower volatile organic content (VOC). Solvents are required in coatings to, among other things, allow the inherently viscous materials which comprise the coating formulation to be applied in a manner that results in a continuous thin film that will harden or cure with the required appearance and physical properties. No single approach to reducing the solvent content in two component epoxy coatings has been found which results in a product with the high degree of performance in different applications that typify the traditional, high VOC products.
One method of lowering VOC is to replace some of the solvent with water. This approach has not been without drawbacks. They include an increased sensitivity of water-borne epoxies to water and corrosive environments, and relatively short pot lives.
It will also be appreciated by those skilled in the art that replacing a substantial amount of solvent with water does not result in a true solution of the film forming components of an epoxy coating. To prevent phase separation and maintain a dispersed state of colloidal dimensions, it is necessary to impart an energy barrier to the agglomeration of the colloidal particles. There are two generally recognized means to accomplish this. The first is to surround the particles with electrically charged species of like sign. In water-borne epoxy coatings it is possible to incorporate charged species with the use of ionic surfactants, but more commonly this is accomplished by adding a compound of sufficient acidity to react with the amine to form a substantial equilibrium concentration of alkyl ammonium ion. Acids such as acetic acid and the like are frequently employed. Such an approach is employed in U.S. Pat. No. 4,246, 148; U.S. Pat. No. 4,539,347; U.S. Pat. No. 4,608,405 and U.S. Pat. No. 5,246,984. Adding acids such as acetic acid or increasing their use level in some cases can also enhance the pot life of a water-borne epoxy, probably either by slowing the overall rate of the amine/epoxy reaction, or by imparting additional colloidal stability. In some cases, the ammonium containing curing agent is combined with already emulsified epoxy resins such as those described below, or in some cases the ammonium containing curing agent is used to directly emulsify the epoxy resin. Unfortunately, water-borne epoxy coatings made by this approach do not have the same degree of water and corrosion resistance of traditional epoxy coatings. Also, systems that rely on the ammonium containing curing agent as the primary emulsifier of the epoxy resin tend to suffer from quite short pot lives.
The other general method of imparting colloidal stability in an aqueous environment is to surround the particles with polymeric chains, such as polyethylene oxide chains, which have a high degree of water solubility. One way of practicing this method of stabilization is to add a conventional nonionic surfactant to the epoxy resin. There are commercially available products that consist of a pre-emulsified combination of low molecular weight (liquid) epoxy resin and nonionic surfactant, or a similar combination which is emulsified by the resin user. Sometimes, special block copolymer surfactants are employed that are designed to have one block highly compatible with the epoxy resin employed, such as described in U.S. Pat. No. 4,446,256.
A different method for nonionic stabilization can be employed as disclosed in U.S. Pat. No. 4,315,044 and U.S. Pat. No. 4,608,406. The diglycidyl ether of a poly(alkylene oxide) diol is incorporated in the epoxy resin advancement of a diphenol and a di- or polyglycidyl ether. In this way, water soluble chains become chemically attached to the advanced, solid epoxy resin, which is then converted into an aqueous dispersion by the addition of water and co-solvents and the application of shear.
Also known in the art are water-borne poly(alkylene oxide) epoxy hardeners with chemically attached nonionic emulsifying chains. A hardening agent for an aqueous epoxy resin composition comprising a reaction product of (a) at least one polyepoxide compound, (b) at least one polyalkylene polyether polyol and (c) at least one compound selected from the group consisting of aliphatic, cycloaliphatic, and heterocyclic polyamines is described in U.S. Pat. No. 4,197,389.
A curing agent for epoxy resins showing good water compatibility is described in DE 4,206,392. It consists of (A) polyamidoamines obtained by polycondensation of (a) dicarboxylic acids that contain oxyalkylene groups or their derivatives with (b) polyamines that contain at least two amino groups condensable with (a), (B) polyamines with at least two secondary amino groups and (C) adducts from (c) polyepoxide compounds and (d) polyalkylene polyether polyols.
A water-borne polyamide curing agent which is made by reacting a polycarboxylic acid with polyamines is described in DE 2,519,390. At least 10 mole% of the polyamines are poly(alkylene oxide) amines.
Water-borne epoxy curing agents which are essentially adducts of diglycidyl ethers of polyethers with polyamines are described in GB 1,326,435. Exemplary amines are the polyethylene amines.
U.S. Pat. No. 5,032,629 describes a hardening agent for epoxy resins which is prepared in two steps. In the first step at least one member of the group consisting of polyalkylene polyether monoamines and diamines and polyamines with a mean molecular weight of 148 to 5000 is reacted with at least one member of the group consisting of diepoxy compounds and polyepoxy compounds in a ratio of hydrogen atoms bound to nitrogen and capable of reaction with epoxide to epoxides of di- or polyepoxy compounds of 1:1.4 to 6. In the second step, at least one member of the group consisting of primary and secondary aliphatic, araliphatic, cycloaliphatic, aromatic, and heterocyclic mono-, di- and polyamines is reacted with the product of the first step at a ratio of reactive epoxide groups to hydrogen atoms on nitrogen of 1:2 to 10. Example 10, according to calculations, shows reacting a diamine with the first step product of Example 7 in a ratio of reactive epoxide groups to hydrogens on nitrogen of about 1:21.